Acoustic testing method



Jan. 18, 1955 J. M. IDE 2,599,835

ACOUSTIC TESTING METHOD Original Filed Nov. 12, 1943 3 Sheets-Sheet lINVENTOR JOHN M. IDE

BY 7%07/ ATTORNEYJ Jan. 18, 1955 J. M. IDE

ACOUSTIC TESTING METHOD 3 Sheets-Sheet 2 Original Filed Nov. 12, 1945INVENTOR JOHN M. lDE

BY :lflw ATTORNEYj Jan. 18, 1955 J. M. IDE

ACOUSTIC TESTING METHOD Original Filed Nov. 12, 1943 DB AUQVI DISYANCEFROM BOTTOM IN FEIY RE55URE'GRADICNT HYDROPNOHE 2.00 (1P5 DlfiTANCE FROMDOTYOH "1 JOH N M.

5 Sheets-Sheet I5 INVENTOR IDE ATTORNEY United States Patent ACOUSTICTESTING METHOD John M. Ide, Quaker Hill, Conn.

Original application November 12, 1943, Serial No. 510,042. Divided andthis application January 27, 1950, Serial No. 140,921

4 Claims. (Cl. 181-0.5)

(Granted under Title 35, U. S. Code (1952), sec. 266) The presentinvention relates to underwater acoustical testing methods and has as anobject the provision of a method for ascertaining certain acousticalcharacteristics of the bottom of a water-way such as a river bottom orsea bottom, particularly those characteristics which affect thepropagation of underwater sound and which determine the initial phase ofa reflected-wave pattern.

Various other objects and advantages of the invention will becomeapparent from a perusal of the following specification and the drawingsaccompanying the same. The present application is a division of mycopending application Serial No. 510,042, filed November 12, 1943, forUnderwater Sound Generator, now Patent No. 2,495,730.

In the drawings:

Fig. 1 is a rear plan view of a sound generator capable of use incarrying out the present method.

Fig. 2 is an edgewise view looking at the left edge of Fig. 1.

Fig. 3 is a section on the line 33 of Fig. 1 on an enlarged scale.

Fig. 4 is a section on the line 4-4 of Fig. 3 on a reduced scale andomitting all parts except the unbalanced rotor device per se.

Fig. 5 is a reproduction of a pair of superposed records made with apressure-actuated hydrophone.

Fig. 6 is a reproduction of a pair of comparison records made withdifferent types of hydrophones.

Fig. 7 is a diagram showing pressure control connections.

The sound generator comprises in general a pair of oppositely facingsound-generating surface elements, one a substantially rigid basesupporting element 10 in the form of a dished diaphragm preferably ofmetal such as iron and the other a relatively flexible, resilientdiaphragm 11 preferably of steel mounted on the supporting diaphragm 10,together with a driving vibrator element 12 rigidly coupled to thesupporting diaphragm 10 for vibrating the latter. The generator as awhole is of general disk shape, presenting an edgewise aspect having anarea several times less than that of the diaphragm 11 or the rear faceof the base 10. The base 10 which as a whole constitutes a rigiddiaphragm is dished on the side toward the flexible diaphragm to form ashallow chamber 13, Fig. 3, surrounded by an annular mounting rim 14onto which the diaphragm is clamped by means of a clamping ring 15 andscrew bolts 16. A gasket 17 of rubber or other suitable materialinterposed between the mounting rim 14 and the outer margin of thediaphragm, seals the chamber 13. An outer gasket 18 beneath the clampingring compensates for irregularities in the opposing surfaces of the ringand diaphragm, avoids metal-to-metal contact and, together with theinner gasket 17 permits slight movement of the diaphragm between themounting rim 14 and the clamping ring 15 while maintaining continuouscontact between the adjoining parts. An annular shoulder 19 formed inthe base 10 at the rear of the mounting rim serves to lock the boltheads 20 against turning, by engagement with a flat side of the latterthus facilitating the mounting of the diaphragm.

To increase the stiffness of the base element 10 without undue increasein weight, suitable stiffening ribs are provided, one in the form of anoutwardly facing channel element 21 extending diametrically across therear face of the base and the others in the form of radially extendingangle members 22, all rigidly secured to the rear face of the base inany suitable manner as by welding. It will be understood, however, thatsuch stiffening ribs or their equivalent may be cast integrally with thebase element.

The vibrator element 12 is of the unbalanced type. The one used in theembodiment of the invention here shown is what is known as a concretevibrator, usually employed for the stirring or puddling of wet concrete.It comprises a non-rotary casing 23 containing an unbalanced-rotorelement 24 journaled at its ends in antifriction bearings 2526 mountedwithin the casing near the ends of the latter. An unbalanced conditionof the rotor is established through the use of a filling 27 of heavymaterial such as lead, within the tubular rotor element 24 to one sideof the axis of rotation. The rotor 24 is driven by a flexible shaft 37extending away from the vibrator element through a flexible housing orhose 38.

Clamping lugs 28-29 and caps 3031 with throughbolts 32, firmly fix thevibrator element 12 to the base element 10. Countersunk portions 33(Fig. 3) on the inner side of the base element 10 receive the heads 34of the bolts, the shanks of which extend through the clamping lugs andcaps to the tops of the caps where on their threaded ends they receiveclamping nuts 35 and lock nuts 36. A suitable sealing material, notshown, preferably soft solder provides a sealed connection between thebolt heads and the countersunk portions which receive them, and holdsthe bolts in place during assemblage and clamping of the parts together.

This arrangement of the mounting lugs at the ends of the channel-beampermits the vibrator element to be mounted in line with the beam andpartly within the channel, and makes for strength, compactness andsymmetry in the structure as a whole. To afford efficient transmissionof driving force directly from the bearings of the vibrator element tothe base member, the mounting lugs and clamps are arranged to engagewith the vibrator in the planes of the bearings 25 and 26. Thisarrangement also has the advantage that the clamping of the vibratorelement takes place where the casing is braced by the sturdy outer ringsof the anti-friction bearings 25-26 thus permitting a firm, tightclamping of the vibrator without danger of distorting the casing.

To enable the natural frequency of the diaphragm to be varied withoutvariation in mechanical structure or interference with the drivingconnections, means are provided for adjusting the air pressure in thechamber 13 to different values. This comprises an air coupling element39 connecting the interior of the chamber 13 with a flexible pipe 40 forthe admission and release of air.

Any known or other suitable source of air or other gas under pressure,and valved connection of the pipe 39 therewith, not shown may be used.Because of the small volume of the chamber 13 of the sound generator,there would result substantial reduction in pressure of the confined gasupon even an extremely small leakage, and to offset this a suitableballast tank is maintained in communication with the pipe 40 asindicated diagrammatically in Fig. 7. Also as here indicated the pipe 40is maintained in communication with a pressure gauge and exhaust valve.

It is preferable that the diaphragm 11 be pre-forrned into a slightly,outwardly-bulged shape as shown, and it has been found that anadvantageous method of accomplishing this is to do so after thediaphragm is clamped in place and by air pressure applied to theinterior of the chamber 13 to force the diaphragm outward suflicientlybeyond its elastic limit to establish the desired permanent set. Thismethod of shaping the diaphragm has the further advantage of avoidingthe use of forming dies and permitting the diaphragm to be drilled forthe clamping bolts and clamped in place while in the flat condition. Italso avoids the necessity for any substantial allowance in the originaldimensions of the diaphragm for distortion incident to the forming.Guide hooks 41 secured to the base element 10 at the outer ends of thefour stiffening ribs 22 serve to hold the apparatus in a supportingframe not shown and which may be of any known or other suitable formcapable of slight horizontal displacement to permit horizontaloscillatory movement of the base element.

In use, the apparatus is carried by a mine sweeping ship below the hull,preferably lowered into the water through a well or sea chest in theships hull. To reduce resistance to movement through the Water, theapparatus is positioned in a vertical plane parallel to the ships keelso as to move edgewise through the water with the movement of the ship.it is placed a distance of several feet below the surface of the waterdepending upon the frequency of the sound waves to be generated and atleast a quarter wave length. With the flexible driving shaft connectedto a suitable source of mechanical power and the air pipe connected to asource of air under pressure, the apparatus is ready for use. It is notessential to the working of the device to maintain pressure above thatof the hydrostatic pressure of the surrounding water be tween thediaphragm and the base element, a variation in such pressure beingnecessary only for varying the resonant frequency of the diaphragm. Uponactuation of the vibrator element by rotation of its unbalanced rotor,the substantially rigid or stiff base element is set into vibrationwithout flexing and at an amplitude determined by the ratio of masses ofthe eccentric weight of the rotor and the remainder of the base with itsrigidly connected accessories. The base element 10 with its rigidlyconnected accessories on the one hand, and the effective mass of theflexible diaphragm on the other hand, form a pair of weights coupledtogether by the compliance of the flexible diaphragm. Such a combinationhas the name tonpilz in German, but there is no single English word forit. The system vibrates with a single degree of freedom in which thedisplacements of the masses are 180 out of phase. Accordingly atresonance the pressures produced by the outer face of the diaphragm 11and the outer face of the base element 10, are additive, resulting in asubstantially omnidirectional field pattern as from a point source, asdistinguished from the figure eight pattern of a dipole source.

Because of the periodic application of force by the vibrator in alldirections through a complete circle in a horizontal plane, there is, ofcourse, also an edgewise vibration of the generator as a whole, but dueto its general fiat-disk form the aspect areas of its two side edges areso small relatively that disturbance therefrom is negligible. Inasmuchas the oppositely facing, disk-like generating surfaces operate in phaseopposition and are small in diameter relative to the length of the soundWave generated, the net result is in effect that of a point source ofperiodic compressional waves, of a frequency equal to the frequency ofrotation of the eccentric weight of the vibrator and harmonics thereof.The output may be characterized as polyphonic, a word here used todesignate a fundamental frequency accompanied by harmonics the intensityof which decreases with the order. At resonance, that is with thevibrator operating at the natural or resonant frequency of thediaphragm, the greater portion of the energy radiated as sound comesfrom the diaphragm. it is obvious that the base element 10 should bemade as light as possible in order to obtain maximum vibrationamplitudes from the available force, and that for maximum ef iciency thediaphragm should have a natural frequency substantially equal to that atwhich the vibrator element 12 is driven. With the air pressure behindthe diaphragm at substantially that of the hydrostatic pressure of thesurrounding Water, the natural fre quency a given diaphragm is at itslowest. As the pressure in the chamber 23 is increased, resonancebecomes broader owing to damping in the air chamber, while the naturalfrequency of the diaphragm rises owing to stiff ness added by thecompressed air and to tangential or radial tcr on resulting from thestretching of the dianhra m. For example, in one practical embodiment ofntion an increase in air pressure from 5 lbs. per inch to 35 lbs. persquare inch increases the reso- .requency from lOl to l20 cycles persecond, and lOilHZT from 50 to 82 cycles per second.

Becuse of the sturdy structure and mechanical nature of the device.large driving forces may be transmitted etllciency to the substantiallyrigid base element 1! and from the latt to the edgeclamped diaphragm inopposite pha e relation and with minimum undesired modes of vibration sothat the sound pressure wave is almost purely sinusoidal and thestresses are so distributed as to reduce likelihood of mechanicalfailure to a minimum.

One practical embodiment using a pre-formed, dished diaphragm 20 inchesin diameter and approximately oneeighth inch thick, operating in anoctave band of 70 to cycles per second at a depth of about 12 feet fromthe surface of the water, produced an optimum sound-pressure level of168 decibels above .0002 dynes per cubic centimeter at a point nearbottom about 40 feet below the surface and a horizontal distance of 6feet from the source.

Investigations looking to the design and installation of acoustic mines,sound detection devices and the like are made possible throughutilization of the following method in determining the acousticalcharacteristics of the bottom of a water-way that is whetheracoustically soft, hard or transitional. It has been found that thereflections of periodic sound waves from the top and bottom boundingsurfaces of a water-way give rise to standing wave patterns beneath aship-carried sound source and that the sound pressure near the bottom,say within a quarter wave length, may represent a minimum, a maximum oran intermediate position in the pattern depending upon whether thebottom is acoustically soft, hard or transitional. As will be shownlater, these differences may be large.

A method of ascertaining the above-mentioned acoustical characteristicsof the bottom of a river, comprises the setting up of a sound fieldbeneath a ship by operation of a sound generator, such as that abovedescribed, carried by the ship and placed in the water beneath the ship,preferably a distance of a quarter-wave length below the surface, andnot substantially less, although it may be more. During maintenance ofsuch sound field a sound responsive device is raised vertically from thebottom of the water-way from a point substantially directly below thesound generator to near the surface of the water to detect the soundintensity at different vertical distances from the bottom below theship. Preferably the sound responsive device is in the form of ahydrophone and is raised at a substantially uniform velocity while thesound pressure level detected thereby is progressively recorded in anyknown or other suitable manner to produce a record of sound intensityplotted against distance from the bottom. Such a manner of recording isexemplified in the patent to M. M. Kinley 2,210,417, dated August 6,1940. It is from an observation of the variations in intensity atdifferent vertical distances from the bottom thus obtained that thestanding wave pattern giving the location of zones of minimum andmaximum pressures is ascertained, which conditions indicate theacoustical character of the bottom.

Typical records for hard and soft bottom using a pressure actuatedhydrophone are shown superposed in Fig. 5 where the dotted-line curverepresents a soft-mud bottom and the solid-line curve a bottom of hardsandy mud. The ordinates indicate decibels above .0002 dynes per squarecentimeter while the abscissas indicate distance from bottom in feet.

The average pressure level gradients shown by the records are much thesame. The initial phases of the standing wave systems are such, however,that a pressure-actuated hydrophone placed on the soft bottom(dottedline curve) would record 16 decibels lower sound level than thesame unit placed on the hard bottom (solid-line curve) assuming the samesound source in both cases.

Such knowledge is important where advantageous placement is desired forpressure-actuated hydrophones, acoustic mines and similar devices.Similar records obtained with a velocity or pressure-gradient hydrophoneshow a behavior, in the response of such hydrophone, the reverse of thatof the pressure actuated hydrophone, that is, maximum response will beobtained from a velocity actuated receiving unit near the bottom whenplaced substantially directly on a soft bottom or a quarter-wave lengthabove a hard bottom. Comparison records coinciding as to frequency andvariations in depth but made one with a pressure actuated hydrophone andthe other with a pressure-gradient hydrophone, are shown in Fig. 6.Discovery of the above phenomena teaches that advantage can be taken ofthe phase relations of standing waves by the employment of velocityactuated receiving mechanisms in the design of acoustic mines intendedto be actuated by sounds from a ship passing directly thereover, inareas where the bottom is known to be predominantly soft and the mine isto be placed substantially directly on the soft bottom or at thewater-mud boundary. Such devices will be less critically dependent upontheir position near the bottom than sound-pressure actuated units, andtheir response will not be seriously weakened by their being covered orpartly covered by the mud or silt of a soft bottom. Because the standingwave patterns of difiereut frequencies manifest the same initial phasenear the bottom, and because this holds true for a considerablehorizontal distance from the sound source, it will be obvious that evenwith a complex waveform there will be a maximum velocity response at thebottom. On the other hand, in case of a hard bottom the use of pressureresponsive devices is indicated.

It will be clear that a knowledge of the acoustical character andconcomitant physical condition of the bottom of a given water-way may beobtained from records produced as above described.

A convenient method of interpreting such a record curve to ascertain theacoustical character of the bottom is to compare it with a group oftheoretical curves computed from a wide variety of hypotheticalacoustical conditions, the acoustical condition of the bottom over whicha particular experimental record is made being substantially thatrepresented by the theoretical curve most closely matched by theexperimental record.

The invention described herein may be manufactured and used by or forthe Government of the United States of America for governmental purposeswithout the payment of any royalties thereon or therefor.

What is claimed is:

1. The method of ascertaining the acoustical condition near the bottomof a water-way which method comprises directing sound Waves through theWater from a point at a given level below the surface to producereflection from top and bottom bounding surfaces of the water-way andmaintain a constant single standing wave pattern having substantiallyfixed zones of maximum and minimum sound pressure While detectingvariations in acoustic pressure in the one stationary wave pattern alonga line extending upwardly from the bottom and plotting alternate maximumand minimum pressure zones whereby to permit determination of theinitial phase of the reflected wave near the bottom substantiallyvertically below the source.

2. The method of ascertaining the acoustical condition near the bottomof a water-way which method comprises maintaining a standing sound wavepattern by reflection from the top and bottom bounding surfaces of thewater-Way, While progressively detecting and recording static acousticpressures at successive points along a line extending upwardly from thebottom to produce a record of sound intensity plotted against distancefrom the bottom whereby to permit determination of the initial phase ofthe reflected Wave at the bottom.

3. The method of ascertaining the acoustical condition near the bottomof a water-way which method comprises maintaining a single standing Wavepattern by reflection from the top and bottom bounding surfaces, and,detecting and recording static acoustic pressure at successive pointsalong a line extending upwardly from the bottom to product a recordcurve of sound intensity plotted against distance from the bottom forcomparison with a ground of curves computed theoretically from a varietyof hypothetical acoustical conditions representative of differentphysical conditions of the bottom of a Water-way.

4. The method of ascertaining the acoustical condi tions near the bottomof a water-way which method comprises establishing and maintaining afield of standing waves of substantially fixed pattern by reflectionfrom i the bottom, and progressively recording variations in soundpressure in the standing wave pattern at varying vertical distances fromthe bottom to produce a record of sound intensity plotted againstdistance from the bottom.

References Cited in the file of this patent UNITED STATES PATENTS1,194,376 Furber Aug. 15, 1916 1,968,448 Harrison July 31, 19342,043,984 Alder June 16, 1936 2,210,417 Kinley Aug. 6, 1940 2,394,461Mason Feb. 5, 1946 2,534,437 Ginzton Dec. 19, 1950 2,556,299 Scott June12, 1951 2,625,460 Cloud et a1. Jan. 13, 1953 OTHER REFERENCESLiterature-A Sound Source for Investigating Microphone Distortion, byWilliam D. Phelps, Journal of Acoustical Society of America, volume 11,October 1939, pp. 219-221.

